1 Field of the Invention
The present invention relates to improvements in an optical system for mixing two light beams having different frequencies, and thereby producing a beat frequency which is considerably lower than the frequencies of the light beams mixed with each other.
2 Discussion of the Prior Art
In the art of optics, it is generally considered difficult to detect a change in the frequency of a radiation, since the detection requires an optical sensor which has an extremely high response. In the light of this recognition, it is desirable to use a light beam having a low beat frequency whose change or variation can be detected by an ordinary optical sensor.
On the other hand, there is proposed a horizontal type Zeeman laser source, wherein a laser tube is subjected to a magnetic field in the axial direction, to generate according to the zeeman effect two laser beams having frequencies different from each other by 1.8 MHz, such that the two laser beams travel along a common axis, so that an optical output having a beat frequency is produced by interference between the two laser beams.
It is also proposed to utilized an acoustooptical modulator which is capable of bending or deflecting an incident light beam, while recognizing the fact that the frequency of the deflected light beam is slightly changed with respect to the frequency of the incident light beam. More specifically, a laser beam having a beat frequency can be obtained due to interference between two laser radiations which are transmitted through a pair of acoustooptical modulators. An example of such a system is illustrated in FIG. 8, wherein a laser radiation produced by a laser source 100 is split by a polarizing beam splitter 102, into a P-type polarized light beam, and an S-type polarized light beam. The P-type polarized light beam is incident upon acoustoopical modulator 104, while the S-type polarized light beam is incident upon another acoustooptical modular 105 via a mirror 103. The light beams incident upon the respective acoustooptical modulators are deflected by these modulators, and the frequencies of the deflected light beams are shifted according to the angles of deflection. The frequency-shifted P-type light is reflected by a mirror 106 and is transmitted through a beam splitter 108. In the meantime, the similarly frequency-shifted S-type light beam is reflected by the beam splitter 108. The P-type and S-type polarized light beams which have been frequency-shifted in different degrees are mixed or combined with each other by the beam splitter 108, whereby an interference takes place between the two frequency-shifted light beams, thereby producing an optical output having a beat frequency which is equal to a difference between the frequencies of the two light beams.
However, the known frequency shifters described above encounter difficulty to change the beat frequency to a desired value. Stated in detail, the horizontal type Zeeman laser source discussed above is only capable of producing a fixed beat frequency of 1.8 MHz, since the difference between the frequencies of the laser beams generated by the Zeeman effect is limited to 1.8 MHz. In the optical frequency shifter utilizing acoustooptical modulators as described above, the coaxial relationship between the P-type and S-type polarized light beams tends to be lost, when a difference between the angles of deflection of the light beams by the modulators is relatively large. Namely, the optical paths of the P-type and S-type light beams may more or less deviate from each other. This tendency increases with a distance between the modulators and the point of mixing of the two light beams. To allow for proper mixing of the P-type and S-type light beams, the difference between the angles of deflection of the light beams by the acoustooptical modulators should be limited to within a fixed narrow range, where the range of the beat frequency that can be obtained is limited.